Typical gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that draw fuel from the fuel tank and deliver pressurized fuel to the fuel manifolds and fuel nozzles in the engine combustor via a main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include at least a metering valve and a pressurizing-and-shutoff valve downstream of the metering valve. In some systems, three pumps are used to deliver pressurized fuel. These pumps may include an aircraft or tank level pump, a boost pump, and a high pressure pump. The boost pump is typically a centrifugal pump and the high pressure pump is typically a gear pump, though in some applications the high pressure pump may also be a centrifugal pump.
Most fuel supply systems are controlled based on the principle that fuel flow is directly proportional to the product of the metering valve area and the square root of the pressure drop across the metering valve. There are some exceptions to this control scheme, such as systems that are based on direct volumetric delivery of fuel. Given the fundamental physics of controlling fuel flow by varying metering valve area and maintaining the pressure drop across the metering valve, the method of varying the area results in moving or displacing a valve in a manner that results in flow area varying exponentially. The manner in which pressure drop is typically controlled depends on whether the high pressure pump is a gear pump or a centrifugal pump. Nonetheless, the high pressure pump is sized to have some excess flow capability at all times.
If the high pressure pump is a gear pump, then the fuel supply system typically includes some type of bypass subsystem maintain the pressure drop across the metering valve and to recirculate excess flow back to the inlet of the high pressure pump. This flow recirculation puts more work into the fuel, thereby increasing its temperature. In some systems, this temperature increase may be in the range of 20-60° F. If the high pressure pump is a centrifugal pump, the metering valve pressure drop is controlled by throttling flow, choking, or otherwise restricting the output of the pump. This also typically results in increased fuel temperature, as more work is being put into the fuel.
Fuel supply systems that use a variable displacement piston pump to regulate flow across a metering valve typically have two operational modes: normal mode and shutoff mode. During normal mode operation, the system supplies fuel to the engine combustor as a function of metering valve flow area. The variable displacement piston pump provides flow to the metering valve to maintain the pressure drop across the metering valve. Since engine nozzle backpressure is a function of metered flow, the discharge pressure of the variable displacement piston pump varies as a function of metered flow to insure a constant pressure drop across the metering valve. Hence, during normal mode operation the variable displacement piston pump operates in accordance with a variable discharge pressure, variable flow scheme. During shutoff mode operation, a shutoff valve is closed, thereby terminating fuel flow to the nozzles; however, the variable displacement piston pump may still be driven. As a result the pump may be driven to maximum displacement and flow. This in turn may cause the system to bypass flow to a pressure relief valve, dumping a great deal of waste heat into the system.
Each of the aforementioned fuel supply system architectures exhibit an unwanted fuel temperature increase. In modern engine and airframe environment, fuel may also be used as a heat sink for the engine oil and other aircraft heat loads, such as environmental controls, electric power generation, and others. It is desirable from a thermodynamic point of view to transfer as much of the waste heat from these heat loads into the fuel as is possible so that the engine turbines can extract that energy. However, in many instances the waste heat from these heat loads exceeds the capacity for the fuel, without overheating the fuel. It is thus becoming increasingly desirable to minimize the self-heating of the fuel system.
Hence, there is a need for a gas turbine engine fuel supply system that includes a variable displacement piston pump that may be operated in an alternative mode, other than variable discharge pressure/variable flow, during shutoff mode operation of the system. The present invention addresses at least this need. There is also a need for a fuel supply system that minimizes the self-heating of the fuel, and thereby provides a thermodynamically efficient architecture.